1. Field of the Invention
The invention relates to a process for preparing alkyl tert-butyl ethers (abbreviated as RTBE below, where R represents alkyl) and di-n-butene from field butanes in a coupled production, isobutane being converted into alkyl tert-butyl ether and n-butane being converted into di-n-butene, the ratio of these two products being able to be controlled by setting the ratio of n-butane to isobutane appropriately by isomerization.
2. Background of the Invention
RTBE are used as additives to gasoline to increase the octane rating. They are typically prepared by addition of alkanols to isobutene, which is also termed etherification. The isobutene can originate from four different sources- from steam crackers, propylene oxide plants, petroleum refineries (le. FC crackers) and plants for the dehydrogenation of isobutane (cf. R. A. Pogliano et al., Dehydrogenation-Based Ether Production--Adding Value to LPG and Gas Condensate, 1996 Petrochemical Review, DeWitt & Company, Houston Texas). In the first three sources, the isobutene arises as a constituent of the C.sub.4 fraction, that is as a direct byproduct. In the dehydrogenation of isobutane, isobutene is frequently a secondary byproduct of such plants, since the starting material isobutane is likewise obtained as a direct byproduct in steam crackers and petroleum refineries or by isomerization of n-butane, which itself is a byproduct in steam crackers and petroleum refineries. The current world production of RTBE is around 25 million metric t/year, with an increasing trend. The production of butenes and butanes as byproducts in a particular cracker or a particular petroleum refinery is too small to be able to exploit completely the "economies of scale", which are latent in the RTBE process. Therefore, isobutene and/or isobutane (for dehydrogenation) would have to be collected from crackers and/or refineries, in order to be able to operate an RTBE plant at optimum capacity. Alternatively, sufficient C.sub.4 fraction could be collected from such plants and these could be worked up on site to isobutene and isobutane. However, opposing both variants, and in particular the second, is the fact that the transport of liquid gases is expensive, not least because of the complex safety precautions.
The term dibutene is applied to the isomeric mixture which, in addition to higher butene oligomers, is formed by dimerization and/or codimerization of butanes, ie. of n-butene and/or isobutene, in the oligomerization of butanes. The term di-n-butene is applied to the dimerization product of n-butene, ie. 1-butene and/or 2-butene. Significant components of the di-n-butene are 3-methyl-2-heptene, 3,4-dimethyl-2-hexene, and, to a minor extent, n-octenes. Di-isobutene is the isomeric mixture which is formed by dimerization of isobutene. Di-isobutene is less highly branched than dibutene and this in turn is more highly branched than di-n-butene.
Dibutene, di-n-butene and di-isobutene are starting materials for preparing isomeric nonanols by hydroformylation and hydrogenation of the C.sub.9 aldehydes thus formed. Esters of these nonanols, in particular the phthalic esters, are plasticizers, which are prepared to an important extent and are primarily used for poly(vinyl chloride). Nonanols from di-n-butene are linear to a greater extent than nonanols from dibutene, which in turn are less branched than nonanols from di-isobutene. Esters of nonanols from di-n-butene have application advantages over esters from other nonanols and are therefore particularly in demand.
n-Butene is obtained for the dimerization, just as is isobutene, from C.sub.4 fractions, for example, as arise in steam crackers or FC crackers. The C.sub.4 fractions are generally worked up by first separating off 1,3-butadiene by a selective scrubbing, e.g. with N-methylpyrrolidone. Isobutene is a desirable and particularly valuable component of the C.sub.4 fraction, because it may be chemically reacted, alone or in a mixture with other C.sub.4 hydrocarbons, to give sought-after products, e.g. with isobutane to give high-octane isooctane, or with methanol to give methyl tert-butyl ether (MTBE), the most important RTBE. After the reaction of the isobutene, the n-butanes and n-butane and isobutane remain behind. However, the proportion of n-butene in the cracked products of the steam crackers or the petroleum refineries is relatively small. In the case of steam crackers it is in the order of magnitude of barely 10 percent by weight, based on the principal target product ethylene. A steam cracker having the respectable capacity of 600,000 metric t/year of ethylene therefore only delivers around 60,000 metric t/year of n-butene. Although this amount (and that of the isobutenes) could be increased by dehydrogenating the approximately 15,000 metric t/year of n-butane and isobutane, which arise in addition to the n-butanes, this is not advisable however, because dehydrogenation plants require high capital expenditure and are uneconomic for such a small capacity.
Isobutene is, as stated, a sought-after cracking product and is therefore generally not available for the isomerization to n-butene. The amount of n-butanes which a steam cracker or petroleum refinery produces directly is not sufficient, however, to produce sufficient di-n-butene for a nonanol plant of a high enough capacity that it could compete economically with the existing large-scale plants for preparing important plasticizer alcohols, such as 2-ethylhexanol. Propylene oxide plants are, as already stated, less productive still. n-Butanes would therefore have to be collected from various steam crackers, refineries or propyleneoxide plants (or C.sub.4 fraction from various sources worked up to n-butene) and the combined n-butene oligomerized in order to cover the dibutene requirement of a sufficiently large economical nonanol plant. However, the transport of liquefied gases is expensive, as already mentioned.